1. Field of the Invention
The invention relates to pitch change systems for aircraft gas turbine engine counter-rotatable propellers and, more particularly, to hydraulic systems for pitch change actuation of counter-rotatable propellers.
2. Description of Related Art
One type of aircraft gas turbine engine includes a power turbine driving counter-rotatable propeller rotors and the power turbine is powered by a gas generator. A pitch change actuation system including a hydraulic fluid delivery system is used for pitch change actuation of counter-rotatable propellers mounted on the counter-rotatable propeller rotors. The pitch change system includes pitch change actuators mounted in counter-rotatable frames which also support the counter-rotatable propellers and blades.
U.S. Pat. No. 4,621,978 by Alan R. Stuart, which issued Nov. 11, 1986, is entitled “Counterrotating Power Turbine”, and is assigned to the General Electric Company, the same assignee as the present patent application, discloses using slip rings to transmit the hydraulic fluid between a static (non-rotatable) engine support structure and the counter-rotatable propeller rotors. Slip rings are also devices used to transfer hydraulic or other fluids or hydraulic pressure from a non-rotating or stationary element to a rotating element in a radial direction. The small area provided by the slip rings to transfer a required volume of hydraulic fluid or oil to both rotating frames may be insufficient at a certain pressure.
Thus, it is highly desirable to provide a hydraulic fluid transfer that has greater and better hydraulic transferring capability between static (non-rotatable) engine support structure and counter-rotatable propeller rotors than that of slip rings.